Fluoroelastomer copolymers based on vinylidene fluoride, for example copolymers with hexafluoropropylene and, optionally, tetrafluoroethylene, are well-known, commercially available materials which possess unusually high chemical, thermal, and oxidative stability. They are readily cured by the use of diamines or by polyhydroxy compounds in combination with phase transfer agents such as quaternary ammonium or phosphonium salts and metal hydroxides. These crosslinking processes are possible because of the inherent susceptibility of vinylidene fluoride-containing polymers to attack by base. However, this sensitivity to base attack necessarily limits the utility of such fluoroelastomers to applications not requiring high resistance to base. Likewise, peroxide-curable fluoroelastomers based on vinylidene fluoride that also contain a peroxide-reactive curesite monomer, such as are described by Apotheker and Krusic in U.S. Pat. No. 4,035,565, are similarly reactive to basic materials. In practice there are many applications, for example in o-rings and shaft seals of internal combustion engines using aggressive oils and in oil well applications, in which resistance to base is required.
For these uses, special-purpose, base-resistant, peroxide-curable fluoroelastomers have been developed that do not contain vinylidene fluoride at all, (or if present, only in small amounts) and are based instead on tetrafluoroethylene. Such fluoroelastomers require either incorporation of special curesite monomers, or treatment prior to peroxide cure, to create radical-reactive sites. Thus copolymers of tetrafluoroethylene and propylene are known but require a curesite (U.S. Pat. No. 3,467,635) or heat treatment (U.S. Pat. No. 4,148,982) for creation of unsaturation to facilitate peroxide cure, and are often difficult to process. Random copolymers of tetrafluoroethylene, ethylene, perfluoroalkyl perfluorovinyl ethers and a curesite-monomer are described in U.S. Pat. No. 4,694,045. U.S. Pat. No. 3,859,259 and Japanese Examined Patent Kokoku 19325/1985 describe copolymers of tetrafluoroethylene and propylene that also contain vinylidene fluoride comonomer in low concentrations (5-30 mole percent), such that reactivity to base is not as severe as in the more highly fluorinated polymers described above. Prior heat treatment of the copolymer is required to promote peroxide curability (U.S. Pat. No. 4,148,982).
Tatemoto, et al., in U.S. Pat. No. 4,243,770 describe the preparation of fluoroelastomers containing vinylidene fluoride, that are prepared by emulsion copolymerization in the presence of chain transfer agents having the formula RI.sub.n, where R is a perfluorocarbon or chloroperfluorocarbon residue having 1 to 8 carbon atoms, and n is 1 or 2. These polymers contain iodo groups at one or both ends of the polymer molecules, and are crosslinkable with peroxides and a crosslinking coagent.
The objects of this invention are to provide base-resistant fluoroelastomer copolymers of tetrafluoroethylene that have improved processibility and are readily crosslinked by peroxide without the use of expensive curesite monomers or pretreatment steps, and to provide an improved process for preparing said polymers.